All voltage source inverter based variable frequency drives (VFDs) have an AC to DC rectifier unit with a large DC capacitor to smooth the voltage ripple. The DC bus capacitor draws charging current only when it is discharged in to the motor load. The charging current flows into the capacitor when the input rectifier is forward biased, which occurs when the instantaneous input voltage is higher than the DC voltage across the DC bus capacitor. The pulsed current drawn from the AC source is rich in harmonics because it is discontinuous. When the pulsed current flows from the AC source, it creates voltage drop in the power delivery equipment. The voltage drop mimics the pulsed current waveform to some extent. The nonlinear voltage drop causes voltage distortion, which is more problematic than current distortion. The reason is that voltage is shared by all loads and it affects all loads connected in an electrical system. Current distortion has a local effect as it pertains to only that branch circuit which is connected to the non-linear load. The voltage distortion is higher if the source impedance is higher. An ac source having a high-impedance is referred to as a “weak AC system”. Hence, connecting non-linear loads like VFDs to a weak AC system requires more careful consideration than otherwise.
There are many techniques, both passive and active, that are employed to improve the current waveform and reduce the overall current harmonics. The active techniques have an advantage over the passive techniques in size, and performance. The cost of certain type of active techniques can be higher than passive technique.
Evaldo L. M. Mehl, and Ivo Barbi in “An Improved High-Power Factor and Low-Cost Three Phase Rectifier”, IEEE Trans. on Industry Applications, Vo. 33, No. 2, March/April 1997, pp: 485-492, discuss a low cost active circuit. Two versions of the basic circuit are shown in FIGS. 1A and 1B. The topology shown in these circuits employs bidirectional switches that are rated only to handle the harmonic compensation current. This current is typically only about 30% of the current rating of the AC to DC rectifier. The topology also lends itself easy for integration with the drive. The circuit does not support regeneration. Since most HVAC applications do not need regeneration, there is no strong desire to propose the fully regenerative active front end converter system, which is bulkier, more expensive, and occupies large space.
The topology shown in FIGS. 1A and 1B is a three-phase partial boost converter that forces current conduction in phases that do not carry current during a typical six-pulse operation. The boost converter requires a boost inductor and since it is desired not to switch at high frequency to limit the conducted EMI, the size of the boost inductor is bigger than normal. The switching frequency of the switches is kept at 2 times the supply frequency. It will be 100 Hz for a 50 Hz AC source and 120 Hz in case of a 60 Hz AC source.
FIG. 1A shows the original method that needed access to the neutral of input AC source. By introducing a dc bus midpoint configuration the original circuit was modified to the circuit shown in FIG. 1B. This allows for the implementation of the active filter circuit without the need to access the neutral of the AC source.
It is well known that there are six distinct diode pairs that conduct in a typical 3-phase AC to DC rectifier. Each interval lasts 60 electrical degrees. The typical phase current is seen to flow for two back to back intervals of 60 electrical degrees, resulting in 120 degree conduction in one half of the electrical cycle. The other half of the electrical cycle also has a similar interval but during this interval, the phase current direction is opposite to the first half since the input current does not have any DC component in a given cycle.
The above description of the diode pair conduction during the rectifier operation shows that one phase does not conduct when the other two phases are conducting. This opens up an opportunity to do something with the non conducting phase when the other phases are conducting. By employing a switch and forcing current to flow through the non conducting phase via the switch into the midpoint of the DC bus, current can be made to flow through the non-conducting phase. To limit the current flowing into the DC bus midpoint, it is standard practice to employ inductors. At some point, the switch needs to be turned OFF and since the inductor is carrying current, it cannot be abruptly stopped. Turning OFF of the switch causes the voltage across the inductor to rise and similar to a boost converter, the energy stored in the inductor is transferred to the DC bus. By turning ON and OFF the switch at the correct time can result in continuous current to flow in the phases. By selecting the inductor appropriately, the current at the end of the conducting period is made equal to about one-half of the peak value of the rated output current.
In the Mehl and Barbi document the control philosophy has not been discussed and the conduction duration of the switches is manually adjusted depending on the load condition.
A control scheme to control the circuit in FIG. 1B to regulate the DC bus of the VFD was put forth by the authors in Ali I. Maswood, and Fangrui Liu in “A Novel Unity Power Factor Input Stage for AC Drive Applications”, IEEE Trans. on Power Electronics, Vol. 20, No. 4, July 2005, pp: 839-846. This control scheme is illustrated in the circuit of FIG. 2. The final topology along with the control strategy discussed in these documents has significant drawbacks. Some important drawbacks are:                a. The topology shown in FIG. 1B and the control strategy shown in FIG. 2 require that the input AC supply be a “wye” connected three phase AC system. In many industrial applications, the 3-phase AC source could be delta connected. The control scheme will need to be modified to adapt to a delta connected AC system.        b. The control idea shown in FIG. 2 aims to achieve DC bus regulation with low input current harmonics for a wide load range, from 40% load to 110% load. In order to achieve this, a current sensor is used in the DC link. When the load current is large, the size and rating of the current sensor needed increases. In addition, the location of the current sensor that is shown in FIG. 2 can create unwanted inductance in the DC bus structure of a VFD and may need an additional snubber to absorb high voltage transients across the inverter switches of the VFD.        c. From the discussions in these documents it is important to note that the DC bus capacitors that form the DC bus filtering network are required to carry the ripple current that is injected into the AC source to reduce the input current distortion. The size and current rating of the standard electrolytic capacitor will need to be increased to handle the additional ripple current.        
The control philosophy used with the circuit of FIG. 2 aims to maintain DC bus voltage at the desired reference level under a wide load range. When the load is less than the rated load, the conduction angle α is reduced depending on the sensed current Idc. When the load is greater than the rated load, the conduction angle is allowed to increase to maintain the DC bus voltage at the reference level. The document describes a strategy having a minimum value for conduction angle α of 14.9 electrical degrees. On the other hand, there exists no upper limit to the conduction angle. However, for safety reasons, the conduction angle cannot be higher than 60 electrical degrees. The drawbacks of this control strategy are as follows.
At very light load condition, maintaining a minimum conduction angle can result in over voltage at the DC bus terminals. The boost action can make the DC bus voltage reach dangerously high levels that can damage the inverter components and cause high voltage stress across the DC bus capacitors. At higher than rated load level, the current in the input inductor will increase to maintain the set DC bus voltage level. The input AC inductors will need to be designed to handle the extra current. This will make them expensive and larger in size. The current rating of the bidirectional switch will also need to be increased. Finally, the current sensor shown in FIG. 2 needs to be placed in between the inverter section and the DC capacitors. This means that the current sensor needs to be rated for the overload rating of the AC to DC rectifier. Moreover, the negative bus needs to be intercepted to insert the current sensor and this can add inductance to the bus circuit that can result in the need for a higher snubber capacitor across the inverter switches in case the load is a VFD type motor drive system.